1. Field of the Invention
The present invention relates in general to the field of information processing, and more specifically to a system and method for providing level dependent management of bass audio signals.
2. Description of the Related Art
FIG. 1 depicts a magnitude frequency response 100 of an example “non-ideal” speaker and of an “ideal” speaker. The frequency response line 102 of the “ideal” speaker (“speaker”) has a flat frequency response across the entire range of human hearing, generally accepted to be from 20 Hz to 20 kHz. The frequency responses of a few high-end speakers approach the goal of obtaining a flat frequency response 102. However, the frequency response of low- to mid-end speakers do not come close to the ideal flat response. The frequency response of the majority of actual speakers including all low- to mid-end speakers deviates from a flat response in several frequency ranges with the range of human hearing. The frequency response curve 104 represents the frequency response of an example, non-ideal speaker. The most pronounced deviation from the ideal flat frequency response curve 104 is almost always in the lower frequencies, which happens to also be where people most easily notice the non-flat, attenuated frequency response. Most consumers consciously or subconsciously evaluate the quality of a speaker based on the loudness and clarity of low frequency audio output. A louder, clear low frequency (“bass”) response generally evaluates to a perceived “better” speaker. Thus, one solution to compensate for an attenuated low frequency response, as indicated in the frequency response curve 104, is to boost the gain of low frequencies to obtain a flatter frequency response. For low-level input signals, boosting is a viable option. However, the ability of audio systems to boost low level frequencies declines as input signal levels increase. A typical 20 dB/decade frequency response fall-off from a flat response can require a gain boost factor of 10 to return to a flat response. Thus, boosting a high level signal can easily exceed the maximum available power. Additionally, boosting a high level signal can cause circuit components to clip, cause digital-to-analog converters to clip, and can physically damage speakers.
True low frequency response generally requires large, expensive drivers, and mass-market home audio is driven by low cost and aesthetics. The two goals of flatter frequency response and low cost are, thus, inherently incompatible.
The problem of a non-flat frequency response is even worse in televisions, where the speakers are generally smaller and cheaper than even the cheapest separate speakers. Cathode ray televisions are further hindered from achieving flatter low frequency response in the television speakers by the fact that large magnet structures normally used in low frequency speaker drivers would need to be heavily shielded to avoid distorting the video image.
Various solutions have been adopted to compensate for the inherently poor low frequency response of non-ideal speakers. One example solution is a fixed frequency bass management system. FIG. 2 depicts an audio system 200 with a conventional 5:1 speaker configuration. The 5:1 speaker configuration has six (6) separate audio signal channels for driving six speakers. The 5:1 speaker configuration includes a left main speaker 202, a right main speaker 204, a left satellite speaker 206, a right satellite speaker 208, and a front center speaker 210. In addition to other features, the 5:1 speaker arrangement allows an audio design engineer to incorporate sound origination location into an audio soundtrack. The main speakers 202 and 204 and front center speaker are generally larger than the satellite speakers 206 and 208. Thus, the low frequency responses of the main speakers 202 and 204 and front speaker 210 are generally flatter at lower, bass frequencies than the satellite speakers 206 and 208. The 5:1 speaker arrangement also includes low frequency equipment (“LFE”) such as subwoofer bass speaker 212. The subwoofer speaker 212 is designed to have a more ideal response in the 20-100 Hz frequency range. Audio frequencies at or below 100 Hz are considered non-directional because most people cannot discern the location from which sound originates if the frequency is at or below about 100 Hz. Thus, the location of the subwoofer speaker 212 is not critical to the 5:1 speaker arrangement.
The audio system 200 also includes a signal processing system 214 that provides the audio drive signals that drive the speakers to produce sound. The signal processing system 214 is, for example, a television, digital versatile disk player, video cassette recorder, stereo system, or other system that is or includes an audio component.
The audio system 200 includes a fixed frequency bass management system 216. Bass management refers to filtering, routing, and mixing low-frequency content to maximize low-frequency response at a system level. For stereo systems with only two loudspeakers, bass management is not particularly useful because there is nowhere to route the low-frequency content that the speakers cannot reproduce. However, in a typical home theater system, such as audio system 200, the bass management system 216 filters out the low-frequency content from the satellite speakers 206 and 208 and adds the low frequency content to the respective main speakers 202 and 204. In this way, the entire content intended for the satellite speakers is preserved, although it is now being reproduced by a different speaker than originally intended. Since humans cannot normally discern the direction from which a sound comes if it is below about 100 Hz, humans will not notice the difference if the low-frequency content is being reproduced by the satellite speakers 206 and 208 (as intended) or by the main speakers 202 and 204 or the subwoofer speaker 212. Since the subwoofer speaker 212 is dedicated to reproducing low-frequency sound, the subwoofer speaker 212 provides a single point to which all low frequencies can be routed. Thus, the satellite speakers 206 and 208 and main speakers 202 and 204 can be smaller and cheaper and, thus, have less than optimal low frequency response.
During setup of the bass management system 216, a cross-over frequency or frequencies are selected. The cross-over frequencies are the frequencies at which the bass management system 216 mixes and/or routes low frequency audio signals from one speaker to another speaker that is better suited for low frequency sound reproduction. The crossover frequencies can either be fixed values for some defined set of listening levels (e.g. low, nominal, loud), or preferably can be continuously variable as a function of output level. In at least one embodiment, the filter structure and control of the bass management system 216 changes the filter without introducing undesirable audio artifacts (e.g., clicks, thumps, zipper-noise, etc.). Currently, bass management in the fixed frequency bass management system 216 is static. That is, the filtering, routing and mixing is configured once during system setup and then left alone. This situation is true whether the system setup is performed manually by the user, or automatically by the system using technology such as Cirrus Logic Inc.'s Intelligent Room Calibration (“IRC”), ADI's Auto Room Tuner (“ART”), Yamaha's Yamaha Parametric Room Acoustic Optimizer (“YPAO”), Audyssey Labs' MultEQ, and Bose's AdaptiQ, etc.). The cross-over frequency is fixed for all audio signal levels and remains static for the signal processing system 214. This manner of setup is not optimal because the frequency response of the speakers is not constant across listening levels. For example, bass management could be setup to route low-frequency content below 100 Hz from the left and right main speakers 202 and 204 to the subwoofer speaker 212. At nominal listening levels, this setup might be optimal, but, as the volume is increased, the small main speakers may start to distort frequencies higher than 100 Hz. This distortion at high volume could be prevented by increasing the crossover frequency to 120 Hz, but only at the cost of some loss of directionality at lower listening levels.
Television signal processing systems typically have very small speakers with very little low-frequency response, rolling off as high as 200 Hz, and generally no subwoofer speaker. In these cases, bass management does not involve any routing or mixing, just filtering out low-frequency content to protect the speakers. A fixed filter frequency is not ideal because the speakers will be able to safely reproduce lower frequencies at lower volumes than at higher listening levels.
Another solution adopted to compensate for the inherently poor low frequency response of non-ideal speakers is psycho-acoustic bass extension (“PBE”). The theory behind PBE is that humans can be “tricked” into thinking they hear a low-frequency sound by synthesizing some combination of the higher frequency harmonics of the desired low frequency sound and reproducing the harmonics instead of the original frequency. While not as good as the “real thing”, the PBE technique can be surprisingly effective. Implementations of PBE date back at least several hundred years to the use of 20 Hz and 40 Hz pipes in churches to substitute for 10 Hz low frequency sound. Several psycho-acoustic bass extension (PBE) algorithms exist in the market today, such as Waves MaxxBassg by Waves Audio Ltd. with offices in Knoxville, Tenn. and SRS TruBass™ by SRS Labs, Inc. of Santa Ana, Calif.
As with bass management, though, the setup of an audio system utilizing PBE algorithms is static, with a fixed crossover frequency or region where real low frequencies are filtered out and replaced with the synthesized harmonics. The same problem occurs in that the ideal crossover frequency is different for quiet, normal, and loud listening levels.